Macklemore and Ryan Lewis's hit song Same Love, which has become an unofficial anthem¹ of the pro-gay marriage campaign in the US, reflects how many gay people feel about their sexuality.

 

It mocks those who "think it's a decision, and you can be cured with some treatment and religion - man-made rewiring of a predisposition（倾向，素质）". A minority of gay people disagree, maintaining that sexuality is a social construct, and they have made a conscious, proud choiceto take same-sex partners.

 

But scientific opinion is with Macklemore. Since the early 1990s, researchers have shown that homosexuality is more common in brothers and relatives on the same maternal² line, and a genetic³ factor is taken to be the cause. Also relevant - although in no way proof - is research identifying physical differences in the brains of adult straight and gay people, and a dizzying array of homosexual behaviour in animals.

 

But since gay and lesbian people have fewer children than straight people, a problem arises.

 

"This is a paradox⁴ from an evolutionary⁵ perspective," says Paul Vasey from the University of Lethbridge in Canada. "How can a trait like male homosexuality, which has a genetic component⁶, persist over evolutionary time if the individuals that carry the genes⁷ associated with that trait are not reproducing?"

 

Scientists don't know the answer to this Darwinian puzzle, but there are several theories. It's possible that different mechanisms⁸ may be at work in different people. Most of the theories relate to research on male homosexuality. The evolution of lesbianism is relatively⁹ understudied - it may work in a similar way or be completely different.

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